Return springs are necessary to make sure that the lowering fingers of the changer return to rest against the return stop after you have lowered a string. They also keep the lowering finger against the return stop when raising a string. Because the fingers are always referenced to a FIXED stop, tuning problems such as drift and repeatability should be typically non-existent.
If you would like to learn how to adjust your return springs see our "Ask Bud" article: Adjustments to Return Springs.
The drawing below shows how a return spring holds the lowering finger against the return stop.